1. Field of the Invention
The present invention relates to a fixing apparatus in which a fixing film having an elastic layer and a heat generating means provided so as to affect a non-fixing surface of the fixing film are included, and an image forming apparatus such as a copying machine, a laser beam printer (hereinafter, called LBP), a printer, a facsimile machine, a microfilm reader printer, a recorder or the like to which the fixing apparatus in question is applied.
More particularly, the present invention relates to a fixing apparatus of a method for (1) forming and bearing an unfixed toner image corresponding to target image information on the surface of a transfer material (paper, print paper, transfer material sheet, electrofax sheet, electrostatic recording sheet, OHP sheet, glossy paper, glossy film, or the like) in a direct transfer method or an indirect transfer method with user of toner made of a heat melting resin or the like by an image forming process means suitable for electrophotographic recording, electrostatic recording, magnetic recording or the like, and (2) performing a heat fixing process to heat and fix the unfixed toner image onto the surface of the transfer material as a permanent fixed image.
The present invention relates to a low-cost and short-risetime (i.e., short-warmup-time) color on-demand fixing apparatus which is used particularly in a color image forming apparatus.
2. Related Background Art
In recent years, colorization in image forming apparatuses such as a printer, a copying machine and the like advances. As a fixing apparatus which is used in such color image forming apparatuses, a heat roller fixing apparatus which has an elastic layer as the fixing member is well known. Here, FIG. 4 shows one example of the fixing apparatus which uses such a fixing roller having the elastic layer.
A fixing apparatus 101 is structured so that a transfer material P on which an unfixed toner image 105 has been borne can pass a contact nip portion 104 formed between two rollers consisting of a fixing roller 102 and a pressure roller 103 both temperature-adjusted.
When the unfixed toner image 105 passes the nip portion 104, this image is heated and pressed by the fixing roller (hereinafter, simply called roller) 102 and the pressure roller (hereinafter, simply called roller) 103, and thus obtained image is fixed as a finished image on the transfer material P.
Here, thermistors 106a and 106b are respectively in contact with the surfaces of the rollers 102 and 103, whereby the respective rollers 102 and 103 are temperature-adjusted based on the respective temperatures detected by the thermistors 106a and 106b. 
Moreover, the rollers 102 and 103 respectively contain halogen heaters (hereinafter, simply called heaters) 107a and 107b at their centers, and radiant energy generated by the heaters 107a and 107b is respectively absorbed by aluminum core metals 108a and 108b provided inside the respective rollers 102 and 103, whereby the rollers 102 and 103 are heated resultingly.
Elastic layers 109a and 109b made of silicon rubber and each having the thickness of 2 mm are respectively provided around the aluminum core metals 108a and 108b, and coating layers 110a and 110b made of excellent separability and heat-resistance resin such as PFA (tetrafluoroethylene.perfluoroalkylether copolymer/tetrafluoroethylene. perfluoroalkylvinylether copolymer resin), PTFE (polytetrafluoroethylene/tetrafluoroethylene resin), FEP (tetrafluoroethylene.hexafluoropropylene copolymer/tetrafluoroethylene.hexafluoropropylene copolymer resin) or the like are respectively provided on the respective outward surfaces of the rollers 102 and 103 to prevent adhesion of toner, paper flour and the like.
In the nip portion 104, the elastic layer is provided on the side of the fixing roller being the fixing member with which the unfixed toner is in contact, because the surface of the toner image is made uniform as much as possible when this image is fixed. However, in the heat roller fixing apparatus having such an elastic layer as this, a heat capacity of the heat roller itself is large, whereby there is a problem that a time (warmup time) necessary to increase the temperature of the fixing roller 102 up to the temperature suitable for fixing the toner image is prolonged. Moreover, there is a problem that costs of the fixing member increase.
Incidentally, a fixing apparatus of film fixing method which is often used in a black-and-white printer or the like is well known as a short-warmup-time fixing apparatus. Here, FIG. 5 shows one example of the fixing apparatus like this.
In a fixing apparatus 201, a fixing film unit 202 which is structured to bring a heater 204 into contact with a transfer material P through a thin fixing film 203 around film guide 207 and thus perform heating is employed as a heating apparatus.
An endless film of heat-resisting resin having the thickness of, e.g., 50 μm or so is used as the fixing film 203, a separation layer (fluorocarbon resin coating layer or the like) having the thickness of 10 μm is formed on the surface of the endless film, and the heater 204 constitutes a resistance heat generating body on a ceramic substrate. A temperature detecting means 209 is in contact with the heater 204 to detect the temperature of the heater 204, whereby the temperature of the heater 204 is controlled by a not-shown control means to become a desired temperature. Besides, any elastic layer is not provided for the fixing film 203 so as to decrease the heat capacity of the fixing film 203.
Numeral 205 denotes a pressure roller which is disposed on the opposite side of the fixing film unit 202 through the transfer material P, thereby constituting a nip portion 206. Thus, an unfixed toner image 105 is fixed as a finished fixed image on the transfer material P by heat and pressure when the transfer material P passes the nip portion 206.
In the fixing apparatus 201 of such a structure, the heat capacity of the fixing film 203 is very small. Therefore, after turning on the power to the heater 204, it is possible to increase the temperature of the nip portion 206 up to a temperature enabling to fix the toner image in a short time.
However, when the fixing apparatus 201 which uses the fixing film 203 having no elastic layer is used as the fixing apparatus of a color image forming apparatus, the surface of the fixing film 203 cannot cope with the surface of the transfer material P, unevenness due to existence/inexistence of a toner layer, unevenness of the toner layer itself and the like. Consequently, a difference in heat applied from the fixing member occurs between the convex portion and the concave portion on the film.
That is, the heat from the fixing member is well transmitted to the convex portion which is sufficiently in contact with the fixing member, but it is hard to transmit the heat from the fixing member to the concave portion as compared with the convex portion. In a color image, because toner layers of plural colors are used and superposed to achieve color mixture, the unevenness on the toner layer is larger as compared with that in a black-and-white image. For this reason, when the fixing member does not include any elastic layer, uneven brightness on the image obtained after the image fixing was performed becomes noticeable, thereby degrading image quality. Also, in a case where an OHP sheet is used as the transfer material, image quality deteriorates due to poor permeability when the image obtained after the image fixing was performed is projected.
Moreover, when a silicon oil or the like is applied to the fixing member having no elastic layer so that the heat is well transferred entirely to the concave and convex portions of the transfer material and the unfixed toner image, a problem that costs increase occurs, and also a problem that the image obtained after the image fixing was performed and the transfer material are sticky with the oil occurs.
Consequently, the fixing apparatus which constitutes a low-cost color on-demand fixing apparatus by applying the fixing film having the elastic layer to the film fixing apparatus as shown in FIG. 5 is known (for example, see Japanese Patent No. 3051085).
However, such a conventional fixing apparatus includes the following problems.
1) The heat conductivity of the silicon rubber or the like used as the elastic layer of the fixing film is not so high, and a response is deteriorated because many members intervene between the surface of the fixing film and the temperature detecting means of the heater, whereby it is hard for the heater temperature detecting means to control the temperature on the surface of the fixing film. Particularly, it is hard for the heater temperature detecting means to detect that the heat on the surface of the fixing film is absorbed by the transfer material P when it passes the fixing apparatus and thus the temperature on the surface of the fixing film decreases, and besides it takes too long to a response in the temperature detecting procedure.
2) In a case where it is intended that in order to control the driving of the heater and thus control the temperature, the temperature detecting means is shifted from the heater portion to the surface of the fixing film, the inside of the fixing film and the like to detect the temperature of the fixing film; it is impossible to detect the temperature of the heater itself.
Consequently, in a case where the heater is energized and the heat is generated in the state that the rotation of the fixing film has stopped due to some reason, the temperature of the heater excessively increases because a temperature rise gradient of the heater is remarkably higher than that at the position of the temperature detecting means of the fixing film, whereby various problems that the heater itself is cracked, a member for holding the heater is melted down, and the like occur.
3) In a case where it is intended that in order to control the driving of the heater and thus control the temperature, only the temperature of the fixing film is detected, particularly, in a case where it is necessary to rapidly increase the temperature of the fixing film, for example, in case of increasing the temperature of the film from a room temperature state to a fixing temperature, the temperature of the heater becomes high when the temperature of the fixing film is increased to a desired temperature, whereby the member for holding the heater and a grease for securing slidableness between the heater and the fixing film deteriorate. Thus, the problem that a torque of the fixing apparatus becomes large due to long-time use. That is, when the torque of the fixing apparatus is large, the fixing apparatus cannot transport a medium in sheet transportation, whereby the fixing film stops and thus a jam occurs.